A vehicle seat having such a head rest of this type is already known and is described in, for example, Japanese Laid-Open Patent Publications Nos. 10-138813 and 2006-212322. In the technique of Publication No. 10-138813, a pressure receiving plate is disposed in a seat back. The pressure receiving plate is pressed by a body of a passenger sitting on the seat when a back-side collision happens, so that the head rest can be unlocked. Upon unlocking, a spring force acting on a support frame of the head rest is released, so that the head rest can be moved forwardly.
In the technique of Publication No. 2006-212322, a head rest is divided into a front side portion and a rear side portion. The front and rear side portions are connected to each other via an X-link. A drive mechanism of the X-link can be actuated based on a signal from a collision prediction sensor, so as to move the front side portion of the head rest forwardly.
In the technique disclosed in Publication No. 10-138813, when the passenger wildly sits on the seat or when the passenger knees the seat back, the pressure receiving plate can be unexpectedly pressed. Further, in the technique disclosed in Publication No. 2006-212322 in which the signal from the collision prediction sensor or a collision detection sensor is utilized, if a threshold value of the sensor is reduced to increase responsiveness of the sensor such that the sensor can be highly responsive to a low speed back-side collision, possibility of a malfunction of the head rest can be increased.
Thus, there is a need in the art to operate a head rest based on detection signals from two sensors, thereby preventing a malfunction of the head rest and in particular, preventing the malfunction of the head rest even if responsiveness of the sensor to a low speed back-side collision is increased.
Further need is to operate the head rest only when a back-side collision of a vehicle is detected at both sides of a seat.
Further need is to increase the responsiveness of the sensors in order to detect the back-side collision of the vehicle.
Further need is to dispose an acceleration sensor, i.e., one of the two sensors, at a position closer to a vehicle body in order to accurately detect the back-side collision of the vehicle.
Further need is to detect an acceleration closer to a value of an acceleration applied to a passenger sitting on the seat by means of the acceleration sensor.